Land-based and marine compasses all need to handle the issue of dip. In certain locations in the globe, due to the position of the compass relative to the north and south poles, the magnet used in compasses will dip toward or away from the ground. FIG. 1 illustrates the phenomenon of dip. Typical magnetic compasses include a magnet coupled with a compass card. As noted above, at certain latitudes the magnet, and hence the compass card, dip. Specifically, FIG. 1 shows a two-dimensional representation 10 of a portion of the globe with latitude lines 12, 14 and 16. For traditional magnetic compasses, the magnet and the compass card 18 are pivotably mounted on a pin 19, dip downwardly in areas around latitude line 12, and dip upwardly in areas around latitude 16. Around the equator, latitude line 14, the magnet and compass card 18 dip imperceptibly or not at all. If the dip in certain latitudes is too pronounced, the compass card 18 will be at such an angle that viewing the numbers on the card is rendered difficult. FIG. 2 shows a more realistic representation of lines of equal dip 24 found on the globe.
Decoupling the magnet from the card is one attempt to introduce self-balancing to compasses. Referring to FIG. 1, a compass card 20 does not dip at latitude lines 12 and 14, but a decoupled magnet 22 does. There are numerous commercially available magnetic compasses with the magnet decoupled from the card. One type is an orienteering magnetic compass. Such a compass is generally used on land by hikers and others to orient themselves with their environment. One manufacturer of orienteering magnetic compasses is Suunto, of Finland, which makes the MC-2G global compass (FIGS. 4a and 4b). As shown in FIG. 4a, the orienteering compass 40 includes a compass card 42, a magnet 44, a magnet holder 45 with trunnions 46, a card case 48, and a pair of jewels 50, 52. The magnet holder 45 encircles the bar magnet 44 and the trunnions 46 hold the magnet 44 to the card 42. The jewels 50, 52 allow the card 42 and magnet 44 to freely swing. FIG. 4b shows an alternative orienteering compass 60 that includes a bar magnet 62 held to the card 42 via trunnions 46 extending from a magnet holder 64.
An advantage to the orienteering magnetic compasses 40 and 60 is that the magnet 44, 62 is decoupled from the card 42. There are several disadvantages in the use of orienteering compasses in marine environments. One major disadvantage is that to properly function, orienteering compasses must be level, which severely impacts their ability to be used in marine environments. Since orienteering compasses are virtually only land-use compasses, their manufacture is less robust than the manufacture of marine compasses. A marine compass having a card decoupled from a magnet is described in U.S. Pat. No. 6,665,944, which is incorporated in its entirety by reference herein.
Another form of magnetic compass is a manual-balance type. This type of compass is properly balanced to function within a certain magnetic latitude. Weight is added to the compass card to level the card. However, manual balancing of compasses is labor intensive and time consuming. Further, such manually balanced compasses are capable of functioning in only a limited part of the world.
Another type of magnetic compass is a counter-weight type, which utilizes the weight of the compass card itself to counter the dipping magnetic force and maintain the dipping angle within an acceptable range. One manufacturer of counter-weight types of compasses is C. Plath, which makes the Venus® compass 70 (FIG. 5). The Venus® compass 70 lessens the dipping by lowering the magnet from the pivot point of the compass card. Thus, the weight of the magnet compensates for the vertical magnetic force causing the dip and allows the card to reach an equilibrium dipping angle with is generally smaller than would have occurred otherwise.
One disadvantage with the conventional counter-weight type of compass is that to provide sufficient moment for the weight of the magnet to counter-balance the dipping force, the magnet must be moved a fairly substantial distance from the pivot point of the card. Referring to FIG. 3, the equilibrium equation for a compass card is:M=(W)(d)(sin θ)where M is the vertical geomagnetic couple or moment, W is the weight of the compass card assembly, d is the depth of the center of gravity, and θ is the dip angle of the compass card. Thus, to move the depth d of the center of gravity Cg of the compass card assembly 30 (including a card 32 and a magnet 34 which pivot about pivot point P), the magnet 34 must be moved away from the card 32. Such compasses must be taller than other compasses, which adds manufacturing costs and prevents such compasses from being placed in certain locations with limited height.
Another significant issue regarding the use of compasses is that compasses used in marine environments invariably encounter spin. Virtually all compass cards spin under some horizontal vibration frequencies, which are encountered when compasses are mounted on powered vehicles, such as automobiles or motorized boats. The difference in inertia between the compass card and fluid within which the compass card is positioned causes relative movement. The relative movement in turn causes contact at the pivot point that leads to friction that drags the compass card in a circular path. Ultimately, the compass card will spin resonantly at some vibration frequencies. Spinning of compass cards inhibits users from properly reading the orientation from the compass.
Rule Industries, Inc., the assignee of this patent application, manufactures compasses which exhibit no-spin characteristics due to a nearly neutral-buoyant compass card within the compass fluid. The near neutral-buoyancy reduces the contact between the compass card and the pivot that causes circular dragging under vibration. The compasses, however, lack the ability to self-balance. There are no compasses that exhibit the characteristics of no-spin and self-balance.